Geodes are geological rock formations in sedimentary and certain volcanic rocks. They have cavities with internal Crystal formations. In Geology these cavities are called vugs.
The interior contains Quartz Crystals and other deposits. It is common to see more than one kind of deposit.
Isn't it a cool thing to have a rock from the other end of the globe in your hands?
Always handle rocks with care. Keep them away from little ones so they don't get hurt in any way.